Elevator cars are supported by wire hoist ropes that each attach at one end to the car, pass over a drive sheave and other guide sheaves, and attach at an other end to a counterweight. As the ropes pass over the drive sheave and guide sheaves, grooved recesses in the sheaves keep the hoist ropes spaced apart and ensure proper operation of the hoist system. The ropes terminate in thimble rods that attach, via a hitch plate, to the crosshead of a frame which supports the car.
Helical springs are typically mounted on the thimble rods and engage the hitch plate attached to the crosshead. The springs minimize the differences among the individual rope tensions due to variations in the groove diameters of the drive sheave and other guide sheaves, and isolate any vibrations transmitted to the carframe from the hoist ropes.
Typically, the springs used between the thimble rods and the hitch plate address both the tension equalization and the vibration isolation concerns. It is known that helical springs with small outside diameters (two inches or less) are best suited for use in equalizing rope tensions. These small outside diameters provide for a stiff spring, which is preferred for a tension equalization application. It is also known that springs of larger outer diameters (over three inches) are best suited for use in isolating vibrations from the hoist ropes. These larger outer diameter springs are less stiff than the smaller outer diameter springs, and are best suited for absorbing low frequency vibrations. Usually, the springs chosen for use between the thimble rods and the hitch plate have an outer diameter of three inches or greater. This spring size is chosen as a compromise between the tension equalization and the vibration isolation concerns.
This outer diameter of the springs necessitates that the holes in the hitch plate for the thimble rods are provided with enough space apart from each other so that the springs do not interfere with each other. However, the width spanned by this hole pattern in the hitch plate typically is larger than the width of the hoist rope grouping coming off the drive sheave and guide sheaves. Therefore, when the thimble rods are placed in the holes in the hitch plate, the hoist ropes are forced to "fan out," with the narrow portion of the fan near the sheave, and the wider portion of the fan at the hitch plate.
Beyond the use of springs, efforts have been made to further dampen the connection between the hoist ropes and the car to prevent transmittal of rope vibration and noise to the car and its occupants.
One elevator hitch assembly uses hoist ropes, thimble rods, helical springs, a hitch plate bolted to the crosshead, a compression plate, and a rubber isolation pad interposed between the compression and hitch plates. The thimble rods pass through the hitch plate, the rubber pad, and the compression plate. The rubber pad is held in place between the hitch and compression plates by bolt heads (the bolts attaching the hitch plate to the cross-head) that nest in recesses in the rubber pad. The rubber pad adds vertical elasticity to the mount assembly and retards horizontal shifting of the hitch and mounting plates relative to each other and to the thimble rods.
However, the problem with using the rubber pad in such a manner is that in order to retard horizontal shifting of the two plates, the rubber must have a relatively high hardness that inherently lessens its ability to isolate the two plates from each other to prevent transmission of vibrations from one plate to the other. The rubber pad thus cannot inherently minimize the passing of vibrations to the car.
Another solution to reduce rope induced noise and vibration includes a hitch assembly that utilizes a plurality of plates which are isolated from each other by elastomeric pads. Thimble rods pass through enlarged openings in the plates and pads, and carry helical springs which engage the lowermost of the plates. A bracket is connected to each of the plates in each pair thereof. The bracket bridges the edge of the isolation pad but does not touch it. The bracket is a thin steel sheet strip and provides a vertically resilient connection between the plates, while at the same time supplying horizontal rigidity between the plates. However, the elastomeric pad is best suited for isolating high frequency vibrations, and is not very effective for isolating low frequency vibrations.